From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 56855C2BB85 for ; Tue, 14 Apr 2020 20:57:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3314320644 for ; Tue, 14 Apr 2020 20:57:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b="Hwu3PZpm" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2633494AbgDNU5W (ORCPT ); Tue, 14 Apr 2020 16:57:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44952 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S2633447AbgDNU5J (ORCPT ); Tue, 14 Apr 2020 16:57:09 -0400 X-Greylist: delayed 62 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Tue, 14 Apr 2020 13:57:08 PDT Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [IPv6:2001:4b98:dc2:55:216:3eff:fef7:d647]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 86AE0C061A0C; Tue, 14 Apr 2020 13:57:08 -0700 (PDT) Received: from pendragon.ideasonboard.com (81-175-216-236.bb.dnainternet.fi [81.175.216.236]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id 13C81521; Tue, 14 Apr 2020 22:57:07 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1586897827; bh=+opjKMJynJ2yyGq/b6mxDNSq3e8f5ZXoGqlv0GJOl4c=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Hwu3PZpm77wy9qhNgn34zcCfb8HT0nsEaRmSyepW5eBqDSjv/PUGzdc0Gw9UzJP7Q 1PlZPJcIoaT11Li8C2XvOTx875+2WLYsx57TjIbr1/JbQ2AHjXV91dvPFkDZJzugNy FufJ5UISqswefu7BV6wUkjGeAC/ZTDCfDlEsCg8w= Date: Tue, 14 Apr 2020 23:56:55 +0300 From: Laurent Pinchart To: Sakari Ailus Cc: Lad Prabhakar , Mauro Carvalho Chehab , Rob Herring , Mark Rutland , Shawn Guo , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , NXP Linux Team , Kieran Bingham , Geert Uytterhoeven , linux-media@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Lad Prabhakar , Maxime Ripard Subject: Re: [PATCH v5 2/5] media: i2c: ov5645: Drop reading clock-frequency dt-property Message-ID: <20200414205655.GO19819@pendragon.ideasonboard.com> References: <1586191361-16598-1-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com> <1586191361-16598-3-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com> <20200406165108.GA7646@kekkonen.localdomain> <20200406173234.GD16885@pendragon.ideasonboard.com> <20200407062241.GA8883@kekkonen.localdomain> <20200407122106.GD4751@pendragon.ideasonboard.com> <20200407151401.GA5206@paasikivi.fi.intel.com> <20200414205552.GN19819@pendragon.ideasonboard.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200414205552.GN19819@pendragon.ideasonboard.com> Sender: devicetree-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: devicetree@vger.kernel.org Hi Sakari, On Tue, Apr 14, 2020 at 11:55:54PM +0300, Laurent Pinchart wrote: > On Tue, Apr 07, 2020 at 06:14:01PM +0300, Sakari Ailus wrote: > > On Tue, Apr 07, 2020 at 03:21:06PM +0300, Laurent Pinchart wrote: > >> On Tue, Apr 07, 2020 at 09:22:41AM +0300, Sakari Ailus wrote: > >>> On Mon, Apr 06, 2020 at 08:32:34PM +0300, Laurent Pinchart wrote: > >>>> On Mon, Apr 06, 2020 at 07:51:08PM +0300, Sakari Ailus wrote: > >>>>> On Mon, Apr 06, 2020 at 05:42:38PM +0100, Lad Prabhakar wrote: > >>>>>> Modes in the driver are based on xvclk frequency fixed to 24MHz, but where > >>>>>> as the OV5645 sensor can support the xvclk frequency ranging from 6MHz to > >>>>>> 24MHz. So instead making clock-frequency as dt-property just let the > >>>>>> driver enforce the required clock frequency. > >>>>> > >>>>> Even if some current systems where the driver is used are using 24 MHz > >>>>> clock, that doesn't mean there wouldn't be systems using another frequency > >>>>> that the driver does not support right now. > >>>>> > >>>>> The driver really should not set the frequency unless it gets it from DT, > >>>>> but I think the preferred means is to use assigned-clock-rates instead, and > >>>>> not to involve the driver with setting the frequency. > >>>>> > >>>>> Otherwise we'll make it impossible to support other frequencies, at least > >>>>> without more or less random defaults. > >>>> > >>>> We're running in circles here. > >>>> > >>>> As the driver only supports 24MHz at the moment, the frequency should be > >>>> set by the driver, as it's a driver limitation. We can then work on > >>>> supporting additional frequencies, which will require DT to provide a > >>>> list of supported frequencies for the system, but that can be done on > >>>> top. > >>> > >>> I guess it would be possible to use different external clock frequencies on > >>> a sensor in a given system but that seems to be a bit far fetched, to the > >>> extent I've never seen anyone doing that in practice. > >>> > >>> Originally, the driver set the frequency based on the clock-frequency > >>> property. If we're removing that but use a fixed frequency instead, then > >>> how is that going to work going forward when someone adds support for other > >>> frequencies in the driver and has a system requiring that, while there are > >>> some other platforms relying on the driver setting a particular frequency? > >> > >> The standard property for this is link-frequencies, not clock-frequency. > >> Deprecating clock-frequency now paves the way to use the standard > >> property later when/if someone implements support for additional > >> frequencies. > > > > The external clock frequency and link frequency are different indeed, but > > they are related. The link frequency has been selected in a way that it is > > possible to generate that exact frequency using the chosen external clock > > frequency. If you change the external clock frequency, chances are good > > there is no PLL configuration to generate that link frequency. > > But aren't we supposed to pick the clock frequency based on the link > frequency specified in DT ? > > In any case, this policy needs to be carefully documented. And by this I mean in a central place, not leaving it to individual bindings. Maxime, we've previously discussed this issue privately on IRC, what's your opinion ? > >>> Although, if you're saying that this driver only needs to work with DT that > >>> comes with the kernel and you don't care about DT binary compatibility, > >>> this would be fine. > >> > >> I believe this series to not break backward compatibility, as the driver > >> only works with a 24MHz clock, so I expect all DTs to specify that. > > > > What you're still doing here is defining the DT bindings based on the > > current driver implementation, not the device properties. > > Quite the contrary, the device doesn't require any particular input > clock frequency, so we're removing that from DT :-) Specifying the clock > frequency in DT is in my opinion a manual workaround for not computing > it at runtime based on the desired link frequency, while the link > frequency is a property of the system as it specifies the range of link > frequencies that are safe to use from an EMC point of view. -- Regards, Laurent Pinchart